1. Field of the Invention
The invention relates to decorative coating using vacuum coating processes for a lower surface coating of glass ceramic cooktops, especially for gas and/or induction applications. In particular, the invention relates to barrier coatings for decorative coatings.
2. Description of Related Art
Lower surface coatings on glass and glass ceramic substrates are currently produced by screen printing processes, among others, which in particular permit to obtain scattering and matte layers. When using luster colors, metallic looking lower surface coatings can be applied which have a silver-metallic glossy appearance.
Metallic reflective surfaces are commonly found nowadays as door fronts in the kitchen area. However, in the field of cooking surfaces, these colors are rather rare, since they either need to be printed using precious metal colors (e.g. Pt), which is very expensive, or because vacuum coating solutions are not sufficiently thermally stable.
Also known are sputtered dielectric interference layer systems in which the color location of the lower surface coating can be varied quickly and easily by varying the layer thickness of a high refractive index layer, without requiring significant process changes. In order to obtain such interference optical systems sufficiently resistant even under the strong thermal effects when used as a lower surface coating (400° C.-500° C.), these layer systems are subject to certain limitations on the possible combinations of materials. For example, the coloring layer, covering layer, and barrier layer are made from the same base material to make diffusion processes visually inconspicuous. This is achieved, for example, using an SiN—Si—SiN layer system. In this case, diffusion processes of the silicon are only slightly perceived visually, if the covering layer is sufficiently thick and remains optically inactive.
Mixed systems of oxides and nitrides are often disadvantageous due to a diffusion of the oxygen into the covering layer, since this results in oxidation and thus a change in color. However, since due to their high refractive indices, oxide layers are ideally suitable as coloring layers (e.g. TiO2 with refractive index n>2.45; Nb2O5 with refractive index n=2.35; Ta2O5 with refractive index n=2.25), the task is to provide a thermally resistant and stable covering layer which is resistant to oxygen diffusion processes to allow for use of new colors for lower surface coatings.
Similar systems based on manufacturing under vacuum work on intrinsic coloring. Known and often mentioned is TiN, which is used in form of a TiN—Ti or a TiN—Ti—TiN layer system, for example. In this case, diffusion processes of the nitrogen into the intermediate covering layer are visually not perceived since TiN has a specific intrinsic color (golden) and so variations in layer thickness are not or only slightly noticeable. However, there is a demand for additional new colors.
Therefore, an object of the invention is to expand the color palette of decorative lower surface coatings while ensuring high thermal resistance of the coating. Even after prolonged thermal stress as it occurs in operation of a cooktop, the color location of the coating should not alter by more than an extent so that the color change is not immediately obvious. Moreover, the coating should be easy and cost-efficient to manufacture.